Control units for regulating pressure or volume flow rate of a compressible or incompressible medium are employed in a variety of applications in process technology. They generally comprise valves or nozzles. The valves can be remotely controlled devices for blocking a flow path or unblocking a flow path for a liquid, gas or vapor, or devices for controlling the flow rate or maintaining constant a pressure.
Depending upon the closing action of the valve member, such valves may differ from one another. Flap-type valves or gate valves close in a direction opposite the flow. Slide valves close in a direction transverse to the flow in a linear displacement of the valve member. Cocks are rotated about an axis perpendicular to the flow direction. The nozzles can differ from case to case as well. They can be venturi nozzles, i.e. nozzles which provide a constriction through which flow is accelerated, diffusers which widen from a small cross section region, or Laval nozzles in which a curved upstream part converges toward a constriction and a downstream part widens away from this constriction also with wall curvature.
In general, these assemblies are connected in control circuits with feedback from sensors of the pressure or volume flow to the controlled elements of the assembly such as the valve members.
When finely divided solids are entrained in the medium, abrasive action in the flow paths of the assembly can cause damage to the walls thereof depending upon the composition and the hardness of the finely divided solids and the velocity of the flowable medium.
To reduce abrasive wear of this type, it is known to provide the flow passage of an engineered ceramic, as described, for example, in at least some of the patents and the application mentioned previously.
An engineered ceramic, as this term is used here, is intended to refer to any of the man-made ceramics which can be of a silicon dioxide and/or aluminum oxide base as well as nitrides and carbides. The following listing is intended to be illustrative of engineered ceramics which may be used alone or in combination, usually in the form of sintered structures: silicon dioxide, aluminum oxide, boron carbide, boron nitride, silicon carbide, silicon nitride, zirconium oxide, nitride or carbide, tungsten oxide, nitride or carbide, titanium oxide, nitride or carbide, magnesium oxide.
Thus in process technology and in metallurgy, where coal, dust ores, finely divided quartz sand and the like are entrained in gas streams, ceramic linings are often provided for the flow passages. The unit of the invention is also intended for these applications.
In flow passages which contain control elements, traversed by a compressible or an incompressible medium, detrimental critical flow conditions can arise, especially when the system operates with high pressure differentials.
The critical flow conditions are gas dynamic or hydrodynamic conditions which can have singularities in the differential equations which define such flows. Because of such singularities, it is not always possible to predict the flow characteristics or to design control units for such flow conditions even with computer design capabilities as they stand at present. In many cases, the differential equations required for such design purposes cannot be ascertained with any precision and may not have solutions when empirical equations are deduced. Critical flow conditions with compressible media are usually associated with flow velocities exceeding the speed of sound and in the case of incompressible media such as liquids, are associated with cavitation and similar effects which generate vapor. Both phenomena are detrimental to the control units or assemblies hitherto employed.